What is this large lump on my upper left side of back that appeared over night? It measures 5"?

... across. Not red or infected. It overlaps my left shoulder blade and is causing mage pain when i use my left arm. I was in a horrible auto accident 3 months ago and one of many traumas to my body were a collapsed lung on the left side. Drain tubes were through out my body relieving me of the fluids filling up inside me